Stable releases are called "Postfix a.b.c", where a is the major
release number, b is the minor release number, and c is the patchlevel.

Postfix 3.4 experimental release

New features are tested in experimental releases. They become
part of the next official release once the code has not changed
for a significant amount of time. Although this code is still
subject to change, it runs on all of Wietse's systems so it is
production quality.

Postfix 2.6 (Final update: February 2013)

Patch
(PGP
signature ) that back-ports the Postfix 2.7 milter_header_checks
feature to Postfix 2.6. This can be used, for example, to control
mail flow with Milter-generated headers that contain indicators
for badness or goodness. For details, see the postconf(5) section
for "milter_header_checks".
Currently, all header_checks features are implemented except PREPEND.

Patch (PGP signature )
to add stress-adaptive behavior to the SMTP server. When some mail
flood keeps all server ports busy, this feature can be used to
quickly drop connections from clients that make errors, and to
reduce the time that Postfix waits for a client command. This may
delay some legitimate deliveries, but it will allow you to still
keep some mail flowing. After the mail flood ends, Postfix reverts
to its normal behavior.

Patch (PGP signature )
to add stress-adaptive behavior to the SMTP server. When some mail
flood keeps all server ports busy, this feature can be used to
quickly drop connections from clients that make errors, and to
reduce the time that Postfix waits for a client command. This may
delay some legitimate deliveries, but it will allow you to still
keep some mail flowing. After the mail flood ends, Postfix reverts
to its normal behavior.